Categories

Archives

Twitter

RSS

I thought of this problem with time travel years ago, and I thought I was pretty clever at the time for noticing it. But the internet has shown me that many authors and sci-fi fans came up with this before I did: It’s impossible to make any slight change to history. The only possible changes are massive ones.

The timeline is a lot more fragile than is usually portrayed in sci-fi. Marty McFly didn’t need to worry about getting his parents back together. He irrevocably erased himself and all of his siblings from the timeline the moment he skateboarded into town and interacted with someone.

Imagine a man gets a great big armful of individual twenty-sided dice, and gives them a toss. They scatter in a particular pattern of numbers. Now, time-travel back to just before he rolls those dice and stop him for a five-second conversation. Will he roll all the same dice in the exact same pattern? Of course not. The ever-so-slight different movement and timing will have him roll a completely new arrangement.

Now apply that same thinking to an even more chaotic event: Conception. Anyone that you interact with in the past will be nudged off of their original rails and onto an imperceptibly new path. This change will be slight only until the moment when they are involved with the conception of a child. Even if the kid is conceived on the same day at roughly the same time (which isn’t a guarantee) it’s going to be a different kid than was rolled in the timeline before you began mucking about. That different kid will live their entire life, nudging everyone else off of their behavior rails, resulting in different children for everyone they meet, and so on.

Poor farmer Bob originally had a son Alan, but thanks to your meddling ends up having a daughter Alice instead. Later, the woman who originally married Alan will marry someone else, and have a completely different slate of kids. Moreover, Alice will likely end up “stealing” a husband from elsewhere in the gene pool, and she will also produce a completely new mix of kids who never appeared in the history you’re familiar with. When the wave of change hits the ruling class you’ll end up with different rulers and wars and different discoveries being made. Every single historical figure born more than a generation away from your arrival will be obliterated, and new ones will appear in their place.

Every time traveler now has unthinkable power. Their slightest action will remove billions of people from the present, and replace them different billions of people. Their every action is re-rolling their future, every moment.

If you time-travel back to (say) 1909, you don’t need to worry about erasing yourself from history by accidentally killing your (N-great) grandfather as he (say) got off the boat on Ellis Island. You’ll obliterate yourself even if you travel to San Fransisco instead of New York, and all you do is bump into someone on the street.

It’s hard to say how small a change you can make, and fiddling around with parts of the timeline becomes a study in chaos theory. What if I warp to an empty pasture in Wales in the mid-1650’s and swipe a single stone off the ground? Will that initiate a ripple effect and overwrite the timeline? What if I swipe a large stone from a wall around someone’s property? What if I bury an out-of-place stone from another part of the world? What if I swat someone’s livestock on the head when they aren’t looking and then poof back to my own time? It seems like you’ll end up with a binary outcome: Your change will either be be noticed and thus alter the behavior of a single human being (and eventually everyone) or it will go unnoticed and have no effect.

You could play around with this and make “sleeper” changes to the past. Locate some ruin that was disconnected from the rest of the world, and leave some change to confound archaeologists. Pompeii is an excellent candidate for this sort of time-traveling prank. Go there just before the eruption of Mount Vesuvius and hand out a few iPods or revolvers or something. Since the city was buried, your historical shenanigans will be concealed for 1,700 years, and the timeline will proceed unchanged in the outside world. From AD 79 to AD 1748 the outside world will proceed along the familiar path, but the moment a digger sticks his shovel into some fragment of plastic or precision-machined metal your changes will suddenly manifest and propagate.

This means pretty much any time travel ends up in a paradox. I dislike this paradox that emerges from any meaningful leap through time from a purely science-fiction point of view. It’s much more interesting to write (or, I suppose, read) about situations where you can travel through time. (The technology required to do so is usually hand-waved by authors and readers alike, but the repercussions of the technology are not. People are funny.)

One way around this would be to eliminate the possibility of a paradox. You could come up with a set of rules where changes to the future don’t have any causality link to the past. So, you can erase your own grandfather (and thus yourself) from the time line, but you won’t instantly vanish the moment you shoot the old boy dead on Ellis Island. You’ll still be standing there, but if you go forward in time you’ll find a different arrangement of people in place, one where your family doesn’t exist and nobody has ever heard of you. This seems to be a common approach to writing about time travel. (Although most stories allow the protagonist to “fix” the changes they made and set things “right” again, which isn’t possible if we take conception into account.)

If we really want to start playing around with scientific buzzwords and write some rules for time travel, we could devise a system where your “changes” propagate at the speed of “time”. (How much this makes sense depends on how much we want to lean towards magic or science for our time-travel ability.) So, if you go back and kill (say) Hitler in 1939 and then jump back to the present, everything will look the same. Your change will be creeping forward through time at the same rate you are, and if you want to see 2009 with your changes in place, you’ll need to wait 70 years. You can stay in 1939 and watch the new history unfold, or you can bugger off to 1 million BC and do your waiting there.

Not that this is “science”, but if you want to sell a setup like this to readers, then you might explain it like this:

Imagine your time machine is simply a device that will teleport you from A to B instantly. If you fire a laser beam at Alpha Centauri and then teleport over there, you’ll still have to wait for your beam to arrive. The time machine works the same way, except with time instead of distance.

This system suggests time is “layered”, and that one dimension of time is contained within another. String theory suggests that there are 11 dimensions, and so you could borrow from that to explain your time layers. If you go back and kill Hitler, you’ll need to travel in the 4th dimension. To see the result in 2009, you don’t just need to go into the future, you need to go into the future of the future. So you’ll need to travel in the 5th dimension. Or wait.

I’m sure a physicist would scoff at this for various reasons, but this explanation is probably good enough to sell the reader on time travel without them putting the book down.

Anyway, use caution if you’re going to be time-traveling. I’m having a pretty good run here and I’d really rather not be erased. Thanks.

EDIT: To be clear, I’m not arguing that time travel IS possible, I’m just musing about effects and rules that would be involved if it was.

1) You can’t change your own past, but you can start a new set of timelines that branch off at the point of your arrival.
2) You change the past, and alter the future to the point where you are not there any more, causing you to not change the past, so you are there. Repeated loops of you existing and not existing occur. Minor variations and ripple effect changes will slowly change things until eventually there’s a loop where you go back to the past, do your stuff, and don’t eliminate yourself from the future. At that point, the timeline carries on in a stable fashion.

It’s not that you can’t change history by travelling back in time but rather than the moment you are able to go back you are effectively splitting your self from your own time line.

Say I go back and I somehow convince JFK not to take that ride. Things will change but I just will never be able to get back to my own timeline. I’m now locked in the one where he didn’t get shot and my timeline continues to exists without me. My loved ones would think I got lost and died somewhere and never hear from me again.

I can stay in the JFK-didn’t-bite-it timeline or I can keep traveling back and forth but I will never be in a place where my parents met and had me or where everything is as I remember it.

It’s a frightening prospect…and yet I would still jump at the chance if I could do it.

Oh good, Im not the only one who puts way too much thought into these kinds of things…

I do want to point out, though, that the theory of “throw a few iPods just before Vesuvius erupts and it wont be noticed for years” is rather optimistic. I see no reason why these “unnoticed” changes couldnt have just as large an effect as your example of pausing someone for 5 seconds.

In the Vesuvius example – people may not consciously notice the iPods under all that ash, but they will still affect things. Say the ash they displace by being there causes a tiny stone to roll down the hill where it wouldnt have had the iPod not been there… That stone causes a bird to fly away 5 seconds earlier… And who knows where that bird might go! Who might see the bird pass by 5 seconds earlier and act 5 seconds earlier or later than they might have done in an alternate timeline?

The classic example of chaos theory is the butterfly causing the hurricane – even the tiniest change in airflow from your very existence could change the course of history.

I… actually, I could go on rambling but Id more energy drinks than is healthy at 11pm. Nice post to incite some brain gymnastics, though.

edit: Oh, on mechanics for timetravel – I quite like the description that time is simply how we perceive changes in state of matter. ie – time is just what we feel when the atoms in our brain move from one place to another. To reverse time, all you need to do is put all the atoms back where they were at that time. To “undo” changes due to time travel you just need to save the current state of all the atoms before you travel. Where you get the power and computation to do that I dont know, but theyre less paradoxes than simply lack of technology.

I also remember a Nova special on time travel mentioning the theory that a time traveling particle could influence its past particle so that it returns to its original course. They illustrated it using billiards:

Snippet: “In order to be self-consistent, a ball emerging in the past must knock itself into the time machine so it can come out again along a trajectory that will knock itself back into the pocket. Novikov wrote mathematical equations for the possible outcomes of a ball hitting itself, and found that a self-consistent solution always exists. This implied that nature would not allow a paradox to arise.”

You’re implying that due to chaos theory, any minute change could cause drastic consequences. Wouldn’t it also be possible that a consequence caused by said change could set off its own chain of events that would eventually nullify the drastic consequences? If any time traveler meddling was nullified by it, what would it say about our universe?

If it transpires that time and existence is one constant chain of events, this could be a different timeline to the one we had yesterday. We wouldn’t know, but maybe time is rewritten every day by clumsy time travelers. We could have come into existence with fully functioning memories five minutes ago as the result of some kind of time-line meddling.

You may think you’re having a good run at the moment, but maybe you’ve only just started to exist.

If that turns out to be happening on a regular basis, I want to know about it, because if I could stop existing at any moment, I want to go home from work early…

Any time travel at all will inevitably have an effect – greater distances just take longer to make changes humans appreciate.

Simply being there will alter airflow patterns, which will eventually muck with weather slightly on some day. It’ll be warmer and someone will sleep a couple minutes longer and boom – everything’s different.

Or slightly change the timing and location of a lightning strike, causing someone to look up when they wouldn’t have.

I have the same outlook as MurrayHewitt stated earlier. This is what I like to call the “Puzzle Piece” time travel theory. It is used in the Hitch Hiker’s Guide series and in Futurama. The idea is the time line that you know exists due to the the pieces of history leading up to it. SO if there were any Time Travel shenanigans they would be included in the puzzle.

Fry goes back in time and kills his grandfather on accident, but then has relations with his grandmother on accident and so his family tree keeps going and he is alive in the normal time line. It all fits together.

If you are alive now, that means no matter what happens time travel wise you wind up alive. Killing Hitler would only give rise to Hiemler.

I recall a Futurama episode where they find an alternate universe inside a box where the only difference was that all the coin flip results have been reversed. Not quite the same as time travel (they tackle that more explicitly in other parts of the series), but it’s the same kind of “minute changes having significant (…well, largely cosmetic changes) differences” thing.

But yes, I think I prefer the… theory-type thing.. where time travel involves hopping into parallel (or was it perpendicular?) dimensions. Mostly because I’m a simpleton Liberal Arts student and it’s the easiest way to handle paradoxes and all their techno-trappings.

I agree with Mike. I think you are misreading Chaos theory and assuming the “possibility” of drastic consequences will always result in large scale changes. Not every flap of a butterflies wings results in a hurricaine.

If Scientific American is correct, the more widely believed model of the “universe” is that there is an infinite number of dimensions with each having an infinite number of different physical laws.

So, it would be impossible to really “change” things because every possible circumstance (i.e. a dimension that is exactly like this one but you have one less atom or a dimension where LOTR is reality but Gimili is actually Shamus) already exists.

I’m a fan of the Novikov self-consistency principle model (I think I got the right one) of time travel: a time traveler can’t change the past but rather fulfills the role that had always occurred. It just feels right, as the timeline would settle comfortably into a non-paradox, consistent state via some kind of negative feedback / process control mechanism.

I like it a lot better when fiction takes this route, because it’s always more consistent and more clever. When it’s not done this way there tend to be inconsistencies in their model.

My favorite example is probably Red vs Blue. SPOILERS FOLLOW! In the second season (right?) Church, who died in the first season and now appears as a ghost haunting a robot, goes back in time to the first season to try to correct the mistakes led to his death. It turns out that he was secretly the cause of every malfunction of the first season, and ultimately caused his own death. The past didn’t change as his time-traveling self was always the cause.

He doesn’t understand the immutable nature of the past and so, at another historical event (I forget what), he runs into hundreds of copies of himself at the same time all trying to prevent this other event. All of them have failed in some way.

From a strictly scientific standpoint, I’m going to just go ahead and say that I don’t buy it. Time travel isn’t possible. I don’t care what anyone says, it doesn’t work. The ONLY way time travel within THIS universe would work is if there is an outside frame of reference from which it is observable. Unless someone can supply direct evidence of that…

BUT from a literary standpoint, it is fun. Time travel makes life interesting. My advice to anyone writing a time-travel plot: decide a priori what your rules are and stick to them. If you later realize you need a rule to change, make it change for the ENTIRE story. People will accept that within the strictures of your story that’s how things work, even if it isn’t really true for the real world. No matter how sciencey you get.

As I alluded to in my post in the previous thread about black/white holes, super string theory allows for baby and parallel universes.

Physics works very hard to avoid paradoxes, maintain causality, and otherwise keep from generating infinite loops and there are many theories as to how this is handled. A leading theory is that an incursion into the past would necessarily spawn a new universe/timeline as “a child process” where the previous timeline continues unaltered while the new thread continues and propagates any sort of changes that may have been created.

All current plausible theories of backwards time travel preserve causality and while allowing matter and individuals to “travel backwards” also requires them to travel an equal or greater distance through space (ie. any method to travel backwards, say, five years, would put you at a MINIMUM distance of five light years, likely greatly more) thus eliminating any sort of impact either direct or via communication (propagates at light speed). Even quantum entanglement, which might though unlikely be able to provide for faster then light communication, would first require the entangled particles to be “slow boated” at less then C to the far distances, eliminating even that as a method to break the rules and send a message back to the past (such as “Mr. President, don’t go to Dallas”) because by the time you get the entangles particles, or get there yourself in order to send the message or otherwise impact the timeline as you knew it to be, the present (the time from where you originated) would have already past by the time you got there and you would find yourself at a LATER time then when you left.

Best case scenario, you are able to travel through a wormhole (which are holes through both space AND time) backwards five years, and it places you at 5 lightyears, the theoretical closest place it could put you. You transmit a radio signal and/or able to at C back to Earth with some message or intent to change something. By the time you return, because you traveled at C, your local time would be at a standstill, but the local time on Earth would continue as normal and you find yourself/your radio message arriving at exactly the same time you originally left, thus preserving causality.

I, for one, prefer that Novikov model, as supposedly used on Lost, as a device for fiction, though I consider it absolute rubbish if we’re to exclude the supernatural.

I’ve noticed a somewhat depressing truth to the multiple-universes-theory: say something awful happens in the time-traveler’s reality that he then goes back to undo. If we assume he can travel backward in time but he cannot jump to the future, or that jumping to the future would only take him through the same line of universes without the waiting, that means there’s still a number of universes out there where the awful thing still happened, and those people have to suffer with it! Aaron Diaz refers to the moment of the time-traveler’s arrival in the past as “splitting the timeline”; the traveler the abandons his old timeline in favor of the one he improves.

My favourite version of time travel is what I’ll call “ripples in the pond”: a time traveller is like a pebble (or rock) thrown into the pond of time; his changes will cause waves and ripples, but always, inevitably, the pond calms and everything is returned to normal. Unless the rock is large enough to disperse the pond altogether (in other words, you travel back in time to destroy the universe), than no matter what change you make, history will eventually “right” itself and return to the path you’re expecting.

So you can change minor details of history, but not the overall march. So if you, say, kept JFK from being assassinated in Dallas, then he would simply be assassinated somewhere else, or he would be impeached, or he would lose a primary race to LBJ, thus setting the country back on the same track anyway.

It’s fascinating to ponder timetravel, but for some reason it seems impossible for me to travel backwards in time. Even with 11 dimensions and parallell realities it just doesn’t feel right. Perhaps it’s the same limitations of my mind that makes me fail to grasp eternity and the size of the universe, but it only seems feasible to travel forwards. I sure don’t have the knowledge to strengthen this hypothesis, it’s just a hunch.

If you can really “change” the past (such that it overwrites the future, or what have you), then I prefer the theory (don’t remember the name) that states that, if such a thing is possible, it will NEVER happen, simply because, if it happens, someone will change the past… and that will keep happening until you get a “timeline” (for lack of a better word) where the time-travel capability is never discovered.

Marauder: the problem with the “must also travel through space” thing is that such travel can be BENT – yes, you travel through space, but (with the help of some strong gravity, for just one example) you end up where you started again (go in a big circle). So that doesn’t prevent anything. Sorry. :-(

Indeed, unless things have changed since I last checked, there were several theories of time travel that did NOT explicitly preserve causuality… and many of the theorists were hoping that some other mechanism would do so.

One great problem I find with a lot of time travel sorts of things is the issue of good old 3-dimensional SPACE – that is, if I were to travel back in time 30 seconds, I would LAND ON MYSELF, as I was sitting right here 30 seconds ago.

Any kind of time travel runs this sort of possibility, though “out in space”, you’re a bit less likely to hit someone. Of course, depending on which theory of time travel you are using, it can be an INHERENT problem (moving one mouth of a worm-hole around at light speed while not moving the other, for instance – EVERYONE who went through would come out at the same moment in the same place – OUCH!).

Fun topic. Well, fun if you are the mood for complicated, solution-less discussion of great complexity. heh. :-)

Krellan: the problem with that theory is the scale of the ripples – all of human history would fit between two of the waves. Sure, things would return to “normal”… humanity will eventually cease to exist or leave any record, and things will be “normal” again, but the mechanisms for that, even if there are any, certainly wouldn’t be fine enough to correct things in the human life time. See the enjoyable but entirely-unrelated-to-the-book movie “The Time Machine” to see an example of how RIDICULOUSLY contrived that becomes almost immediately. The main character tries to save his fiancee several times, and each time, something manages to kill her anyway… the last time he tries, a PIANO FALLS ON HER HEAD. Yeah.

nice discussion going on here! but consider this. i say that time travel will never be possible. let’s consider that i invent a time machine tomorrow. others will come to know of it eventually and more machines will be built. now at least one person will go back in time to some instant. since any person can do this many, many times and there are many many people in the world (not only in the present but the future too), time travel will be known to people of every time period. someone could go to 1997 and tell people there. he could also go to 1297. since we know it to be true that there is no documented case of time travel in history, it means that time travel will NEVER be discovered. so what do you all say? any flaws in my thinking?

@Perry: There is always the possibility that backwards time travel is impossible, but forwards time travel is possible. This also inherently resolves all paradoxes. It is also possible that time travel is possible, but only back as far as the point that the time machine was created. And of course, it could simply be so impractical that it has only ever been practiced by people who succeeded in not being noticed.

1) My great-uncle actually coined the term “time travel paradox” in his short story “Paradox” back in the early 1930s.

2) Both he and Shamus are under-estimating what Chaos Theory actually says about time travel.

Accroding to Chaos Theory, if you can go back in time and it’s possible to change anything, you will irrevocably change the weather patterns of the entire world the instant you displace the atmosphere with your presence. Take a breath and you’ll probably destroy/undestroy a city by altering the course of a hurricane some amount of time later. After that incident, any other specific consequences of what you do become insignificant.

3) Time is most likely unchangable. If you travel back in time, you always had travelled back in time and what you did is what happened anyway. I forget which specific theory this is called, but it’s what makes the most sense. It also means that the future is set in stone because the past isn’t really any different from the present or future.

Given the whole concept of space and time being part of the same thing, this really just makes the most sense. It takes the “fun” out of time travel stories a bit (I feel sorry for my great-uncle), and means that “destiny” is an undeniable hard fact (I feel sorry for the entire scientific community), but it makes the most sense.

it’s going to be a different kid than was rolled in the timeline before you began mucking about

When gamers reproduce…

You can’t change _anything_ when you time travel back into the past – anything you do already happened. Go back in time and murder your grandfather as a young boy? Well then he obviously wasn’t _really_ your grandfather, because he was already dead…

Shamus, have you read Connie Willis’s time travel novel, To Say Nothing of the Dog? It seems to use that Novikov principle other commenters have mentioned, and the Timeline itself is self-correcting (and seems to have some kind of guiding intelligence). And it is great fun!

I’ve had an idea for a short story about time travel running around in my head for about 15 years now. Essentially, a guy goes back in time to try to stop the JFK assassination. However, when he gets there, all he finds are people from the future either trying to stop the assassination, cause the assassination, or just observe it.

Personally, I think that everyone else is a time traveler come to watch the past. Which only tells me that the future is really, really boring.

There’s always the good old predestination theory: No matter what you change in the past, it won’t change the future because those changes have already been factored into the present.

Thus if you go back in time and try to kill hitler, you will inevitably fail, and your actions will be remembered as Operation Valkyrie or something.

It is self-defeating to use predestination when writing time travel fiction, though, because it just ruins every story. Still, sometimes authors make a head-nod to it, or incorporate one aspect of predestination, usually with some kind of loophole to get out of it.

I realized this when I thought about how much of a chance encounter my last girlfriend was. I was slightly sick and did not know if I really wanted to leave the house, it was basically a coinflip. I did, met her and literally spend a year with her. That year would have been incredibly different otherwise.

Awesome post. Your “conception” argument is very interesting; I’m writing some stuff right now and that’s one of the dynamics. Time travel also changes everyone else’s life, not just your own. Minute things will be off because of the “freeze frame” of when you left.

I subscribe to the Terry Pratchett theory of time travel – namely that history has a lot of inertia. You can change the details but you can’t change the big picture (paraphrasing).

“Shoot the dictator and prevent the war? But the dictator is merely the tip of the whole festering boil of social pus from which dictators emerge; shoot one, and there’ll be another one along in a minute. Shoot him too? Why not shoot everyone and invade Poland?”

Regarding Chaos Theory: It’s easy to think of history like a fractal, say the Mandelbrot set. There are parts of the MS where the tiniest change causes large effects, and there are parts of MS where it is solid and large changes in co-ordinates cause no change.

Re Time Travel in General: There are dozens of models, some more fun than others. I’ve always liked Niven’s story where some space-archeologists point out the their government that past civilizations that seemed on the verge of completing time-travel projects (which were large and unsubtle) always collapsed before completing said projects, often from quite unlikely circumstances. They theorized that the Universe had an “immune system” preventing time travel. Said government then *faked* a time travel project so their enemies would try to keep up with a real time travel project and get destroyed.

Alternately, there’s the view of history where some highly jaded yet nigh-omnipotent being at The End Of Time uses time travel to send itself back with highly altered consciousness to various parts of history to have “interesting and authentic experiences.” This being is, in fact, everybody.

Shamus’ theory brings up interesting consequences when applied to existing fiction where time travel works effectively: are there higher powers at work guiding the timeline in an intelligent manner in that universe? (Well, yes…the writers…) Shows like Quantum Leap give a definite yes to the idea of God or Father Time adjusting things, as do most comic books and possibly Star Trek (The Q have to do something when they’re bored).

String theory and M-theory postulate extra, tiny space-like dimensions… what is needed for timeline changes (in the absence of multiverse theory, or perhaps as a supplement to it) is an additional, unbounded (or at least, usably-large) time-like dimension. Just take the idea that space and time are alike, but temper it with the wisdom that they are in some ways un-alike. For instance, we can accelerate in space quite easily (up to a point), but accelerating/deccelerating in time (or teleporting in time, as much time travel mythology apparently indicates) is a neat trick. However, because time is dimensional (like space), we can conceive of a timeline as a sort of “block” (look up block time or 4-dimensionalism.) Gods trying to mess you up. (Seriously, though, my father has made the claim, in all earnestness, that the Tower of Babel was probably a particle accelerator.) Certain rules bear on how objects relate to one another in spacetime… if X is moving at 3 m/s away from Y at time t, then at time t+1 second, assuming no force acts on X or Y, X must be 3 meters further from Y than it was at time t. If the universe is deterministic (as it seems it must be for block-time to be true) then the laws of physics (the “real” ones, not human approximations… yes, huge metaphysical realist bias here…) and the state of the world at a time entail both the history and past of that world. (that is, you can predict, or post-dict, at least in principle, any event in the timeline.)

This isn’t so weird… consider a spacetime block as a like heap of slag. There’s a certain way slag piles up and becomes a heap, and this depends on physical laws and where it’s placed… but if you drop 20 million bits of slag from above a point, you’re going to get a roughly conical heap. Now, making changes in the particular shape of that heap requires, at the very least, time. But to make changes in a spacetime block, we can’t use time. We’re trying to make changes TO time. So, we need hypertime, and background time-like dimension in which block universes exist.

I will admit that Time Travel is one of my favorite debate topics and a plot mechanic I loath now.

*Disclaimer: This theory is based on my limited knowledge of astro-physics, such that we and the universe are spinning through space and since the universe is expanding we are spinning away from the center point.

My main complaint, which Caffeine brought up, is the state of time. The past does not exist, nor does the future, they are states of matter at specific locations, at specific times (time being a measurement we use and not an intrinsic property or force) within the universe. Once the states change (very fast rate), that particular arrangement is gone and thus that moment in time is gone. You can’t travel back in time to say 1776 because all that’s there (most likely) is empty space. The universal arrangement that was the signing of the Declaration of Independence in 1776 no longer exists.

The only way to travel “back” would be to reassert the entire Universe’s state back to match that moment, which I am pretty sure is impossible (would require massive amounts of energy, data on the state and the changes needed to get to it, and teleportaion/matter altering technology), but doing so wouldn’t actually be time travel, it would be popping the universe into that State in the current timeline; wiping out the physical existence of everything the universe has done up to that point. However the actual dates and progression of time(from the Universe’s point of view) have not changed.

Since you can’t time travel anyways, I don’t have to worry about causality. Now to hand wave for a moment, if you could magically somehow do it, there is no logical reason the self-consistency plan would work (since you can’t time travel anyways) therefore I would posit that Shamus is right, any change no matter how small has a chance of significantly changing the future, and probably negating your existence. Hell, you could bump into a person and immediately change from yourself into a completely different time traveling person of the opposite gender who wouldn’t even notice anything had happened. The person you bumped into wouldn’t even notice since from his point of view it was always a woman. It’s as possible as any other change.

That’s something that always irked me, in almost any time travel system that allowed causality, changes should propagate thoroughly, no one, yourself included, would ever notice the change.

I won’t bother attacking multiverse ideas since I lack any psuedo-science to back it up aside from I just don’t think it makes any sense.

If you want a pretty interesting movie about time travel, I’d recommend Primer. All the pseudo-technical talk at the beginning can be a turn-off, but once the time travels start happening the events becomes very hard to follow. By the end, I was completely confused and had to watch it again and read an explanation to finally get what happened.

I think time travel is possible in both directions, but with severe limitations. You can witness the past of your particular location, if you can somehow go faster than light or take a shortcut through space that light cannot. You can witness the past of all other locations, how far in the past depends on your relative distance to them. But you cannot interact with the past or alter it in any way.

The future is the opposite, you can interact with it, by doing something in the present, but you can only guess what will happen, and hope you’ll witness it. The probability it will happen depends on what you know and what’s true of what you know. Sometimes this probability is close to 1 (100% sure), but you’re never safe from anyone or anything altering your prediction by his own actions. The future is anything but set in stone, as the smallest thing can change it radically (see examples from everyone, they apply to present actions on the future). You just can’t know what it will be like for sure.

I’m trying to remember where, but there was an article about a genius working on a different view of time and space.

Physics relies on time and the speed of light as the two great constants. Every other term/measurement can be derived from those two. Relativity, however shows that time is not exactly constant.

His theory/argument was that time is derived from velocity, instead of the reverse. There are no objects in the universe that are not in motion; the best you can do is be still within a given frame of reference- and then everything has the same starting velocity relative to the larger frame of reference.

This would make time travel pretty much impossible though. But I did always find this idea interesting and simpler than requiring string theory.

I highly advise you read Brian Greene’s “The Fabric of the Cosmos” (and its precursor, “The Elegant Universe”). Both tackle quantum mechanics, relativity, and spacetime in ways that the average intelligent but scientifically (specifically physics) ignorant reader can comprehend. “Fabric” has a large section devoted specifically to the possibility of time travel and these so-called “paradoxes”, and outlines exactly why they are illusions of our “common sense” perception of reality.

I’d recommend the same to anyone else attempting to muck about with pseudo-theories about how time travel would “really work”.

I did see Primer, and I did get lost near the end. I got what he was doing technologically, but not WHY. Then I got lost on the minutia. I intended to go and read a synopsis and try to sort it out, but never got around to it.

@NRowan: The thing about A Sound of Thunder is that it relies a bit too much on that hovering walkway-thing as a plot device. Apart from anything else, what did they do when they set it up? Second of all, stepping on a butterfly changes modern-day politics for the worse, but shooting great big dinosaurs has no effect whatsoever? Third, in the story, politics is all that changed. The change was started millions of years before humanity even existed. We’re supposed to believe that humanity STILL EXISTS (essentially, Shamus’s conception-argument applied to evolution itself), let alone has roughly the same people?

As for “history with inertia”, that’s an interesting point. Shooting Hitler won’t simply erase the circumstances that led to World War II. (I once had an idea for a story in which two characters discuss time travel, one of them mentions giving Hitler art lessons so that he’d get into that art academy he was rejected from, and the other one says “Yeah, in this alternate history, Hitler never goes into politics but instead becomes the world’s greatest artist, hailed as a modern Da Vinci, but is tragically killed in the bombing of London.” “What?” “Germany goes on to win World War II, because they aren’t being led by such a damn fool this time around.”) You’d have to go back far enough that the changes would also erase World War I.

As for the 2002 Time Machine movie, someone should make a montage of all the scenes leading up to the woman’s death, and dub “Might is Right but Tight” with the caption “GAME OVER – Press R to reload” over the deaths.

That book “Millenium” had a framework where, if changes were made in history, it took time back in the future.

Also, it’s funny that I, too, had an idea about people going back to assassinate Hitler. Only, someone else had already done that (killed him in his cradle), and they get picked up by the police after spending too much time wandering around asking questions that don’t make any sense and all their future equipment gets confiscated. I don’t know where things go after that, but…

As long as we’re on the subject of time travel, WikiHistory is an amusing short story about a future where time travel is commonplace. It doesn’t follow the chaos-theory guidelines that Shamus mentioned in the article, but it’s a fun read nevertheless.

The butterfly effect is an excelent movie about chaos theory showing exactly that:Even the smallest change can alter the entire universe in ways you cannot imagine.

The theory of parallel universes is the most plausable one when dealing with time travel,and it eliminates all the paradoxes.This way,you are never altering your own timeline,but some very similar one where a different you is being affected.

If you’re in the mood for interesting speculations on this topic, I’d recommend Larry Niven’s essay “The Theory and Practice of Time Travel;” Niven is a sci-fi writer with a good grasp of science and a good sense of humor to boot — it’s been awhile since I read it, but I remember it as being quite interesting, and along the lines of what you’re doing here.

The same volume has a bunch of other similar Niven essays, my favorite being “Man of Steel, Woman of Kleenex,” which details the incredible lengths Superman would have to take to procreate with Lois Lane — quite hilarious.

You can time travel to the future already, I’m doing it right now. It takes me about sixty minutes to travel one hour ahead, though.

But you could do it faster if you travel at relativistic velocities. There are astronauts who are slightly younger than people who were born at the exact same time as they were.

Of course, you can never come back, so it’s not like time travel is usually portrayed in stories.

I’m glad someone brought up the fact that the Earth is constantly rotating and revolving (not to mention that the solar system and the galaxy are moving too), so that if you could travel in only time, you’d end up in airless space. Unless gravity affects time travel somehow.

I think it was Larry Niven who first wrote about the problem with time travel changing the past. If you have a universe where time travel changes the past, the past keeps getting changed, until one of the changes makes time travel impossible, and then the universe stays like that. So the most likely state for the universe to be in is one in which time travel that can change the past is impossible.

I still think it is possible to make small, yet noticed changes in the timeline. If a time travelling dog were to run across my field of vison (and upon leaving it promptly went back to the future without anyone else seeing it) I don’t think it would change any of my actions.

I know that it would minutely change air currents and bend some blades of grass, but those effects are so small that it would probably take thousands of years for those to matter in the timeline.

Any change in the timeline needs to have an associated length of time with it. If I went 5 minutes into the past and shot the queen of England and came back I wouldn’t notice a change. However if I shot the queen of England 50 years ago, then I’m sure I would notice some significant changes upon arriving back to the present.

All actions in the past have some space/time propagation rate, to find a minor change due to time travel you just need to time it so that the amount of time you go back is the “elbow” of the effects vs time curve.

You’re giving the butterfly effect too much credence here, Shamus. We don’t know that history is an inherently unstable system. For all we know, it could already be at a stable point, and no amount of time travel will perturb it from its course. Of course, the first time you time travel, you’ll find out. :-P

I often have to discourage my wife from traveling through time by insisting that it will undo the genesis of our children.

Of course, the way I describe it to her is usually more colorful. Like, “if you throw off by even one second the time I blew my wad in you, then you’ve just murdered our child who has yet to be conceived. However, feel free to travel back in time AFTER the children were born and have sex with me more. A lot more.”

That’s a list of all the IF games that this guy has done, but Shrapnel is among my favorites. I can’t tell you too much about what happens since it’ll ruin it all, but it involves a lot of time travel.

Oh, and if you die the first time around, don’t get too frustrated. ;)

I’d subscribe to the “only forwards” theories, mostly because it feels right (very scientific!).

But to ignore all the phsyics posturing for a moment, if you were able to go back in time Hollywood style, surely you would also be taking the vast amount of microorganisms, that use your body as home, back also. Anything you touch, breathe on, could have transfer of organisms. Very War of the Worlds, but from a slightly different perspective.

You take a trip back to 1909. Whatever you do back then has already been done. It has already happened. So not only can you NOT change history, but your trip back in time was ABSOLUTELY NEEDED for the world to continue as it was supposed to.

Of course, this means that up until you complete the time travel trip back, you have no free will… so let’s think about that for a while.

Perhaps Time Travel/Time Altering is impossible simply because the concept of “time” is meaningless. What if the human concept of “time” is simply the attempt of the brain to quantify an infinite concept that has no finite paralell? Ultimately, if you cannot get to a beginning, and cannot arrive at an end, then the concept of “time” is really irrelavent except as a “local” event management tool. In physics, we talk about multiple dimensions but honestly, is there really a way to describe “travel in the n-th dimension” in a way that our finite minds understand?

I am reminded of a book I read once called “Flatland” by Edwin A. Abbot. Although it was presented in the context of a commentary on Victorian society, the mathematical and “physical” principles behind the story were profound to say the least.

Not saying that the above argument is even a valid one, but although I am fascinated by the discussion of multiple dimensions and space-time theory, I have a hard time saying that that is absolutely the right path to go down for a workable theory…